In current gas turbine engines of the twin spool type the leakage of high pressure compressor discharge air through the labyrinth seal carried between the high pressure compressor rear rotor shaft and diffuser case typically establishes the thrust loads on the component parts such as the thrust bearing. This air is also used downstream in the engine for cooling purposes, particularly, the bearing compartment and the high pressure turbine. In these heretofore designed engine models, the thrust balance is dictated by the diameter of this seal and/or pressures and once the balance is set for a given operating point, it cannot be varied for any other. Hence, the balance point is selected for a predetermined mode of engine operation, and it is a compromised value, as it will vary with all other operating modes. Hence, on many engines it is difficult, if not impossible to provide the desired thrust load at both the sea level take-off condition and also, the cruise condition.
Inasmuch as the temperature surrounding the bearing compartment is dictated by this discharge air, on certain engine designs, particularly those comtemplated for the future, this temperature is unacceptably high. Obviously, since the temperature of this air is higher than the temperature of the air of heretofore known engines, its use for cooling of the high pressure turbine is less than desirable, and when such is used, considerably more flow is required, resulting in a higher thrust specific fuel consumption.
We have found that we can achieve a more effective thrust balance system than the heretofore engines by an addition to the thrust balance seal judiciously locating seals fore and aft of the bearing compartment so as to isolate this area and independently control the atmosphere surrounding the bearing compartment. In accordance with this invention, a controllable valve or orifice adapted to bleed compressor discharge air from the chamber immediately upstream of the fore seal mentioned above serves to control the pressure drop across thrust balance seal so as to effectuate a desired thrust load during a given operating condition of the engine.
The air bled from this chamber, ahead of the fore seal being at a compatible temperature and pressure is routed externally to the high pressure turbine static structure for cooling purposes.
Cooler air, air that is cooler than the environment surrounding the bearing compartment, is fed into this environment between the fore and aft seals, is bled from the mid-stage compressor either through an anti-vortex tube directing the compressor bleed air radially inward to the engine's shaft where it flows axially to the environment surrounding the mid-frame bearing compartment, or externally from the mid high pressure compressor case thru the diffuser case to the bearing compartment, this air then is used to supply cooling to portions of the high pressure turbine rotor.
By virtue of this invention we are able to provide a cooler atmosphere surrounding this bearing compartment and supply cooler air to the high pressure turbine rotor.
The external valve controlling the leakage flow of the thrust balance seal upstream of the fore seal, serves to modulate the thrust balance.
We have also found, with the additional seals that in engines that employ turbine on board injectors (TOBI) we can increase the TOBI discharge pressure with a minimal impact on thrust balance and leakage.